Since the discovery in 1982 (JP-A S59-46008), NdFeB magnets comprising tetragonal Nd2Fe14B compound as major phase (simply referred to as Nd magnets) have been used in a wide variety of applications. Nowadays, they are useful materials in the manufacture of electronic/electric, transportation and industrial equipment. Despite some drawbacks including relatively low Curie temperature (˜310° C.) and poor corrosion resistance, the Nd magnets have advantages including high saturation magnetization at room temperature, relatively inexpensive constituents, relatively high mechanical strength. The Nd magnets surpass the prior art 2-17 SmCo magnets and find ever increasing application fields. Among others, their application as vehicle-mounted components including motors for electric vehicles (EV) and hybrid electric vehicles (HEV) and power generators is considered most promising (JP-A 2000-245085).
Vehicle-mounted components are typically used in an environment in excess of 100° C. In the case of EV and HEV motors, heat resistance at temperatures above 150° C., and sometimes around 200° C. is required. Nevertheless, because of relatively low Curie temperature (˜310° C.), the Nd2Fe14B compound undergoes a substantial decline of coercivity at high temperature (typically a temperature coefficient of Hc of about −0.6%/° C.). It is difficult to use low-Hc magnets in the temperature range in excess of 100° C. As used herein, the term “coercivity” refers to coercivity Hcj of a M-H curve, often abbreviated as Hc.
The most desirable solution to this problem is to improve the temperature coefficient of coercivity. However, an essential improvement is difficult since this solution is based on magnetocrystalline anisotropy constant and Curie point which are inherent physical properties of the magnetic Nd2Fe14B compound. The second best improvement is to substitute a heavy rare earth element Dy or Tb for part of Nd to improve the anisotropy field (sometimes referred to as Ha) for thereby increasing the coercivity Hc at room temperature. The high coercivity Hc at room temperature ensures that even when a decline of Hc occurs upon exposure to a high temperature, a Hc level for the intended use at the temperature is maintained. Not only substitution of Dy/Tb for Nd sites, but also substitution of Al, Cu, Ga, Zr or the like for Fe sites is effective for Hc improvement. However, the Hc enhancing effect by such substitution is limitative. The element that achieves a Hc enhancing effect in proportion to the substitution quantity is limited to heavy rare earth elements Dy and Tb.
As discussed above, the substitution of heavy rare earth elements Dy and Tb is very effective for Hc enhancement. However, since Nd and Dy/Tb produce magnetic moments in inverse directions, the saturation magnetization (sometimes referred to as Ms) decreases in proportion to the substitution quantity. Since a decrease of Ms occurs in exchange for an enhancement of Hc, the maximum energy product (sometimes referred to as (BH)max) decreases in proportion to the squares of Ms (i.e., Ms2). That is to say, heat resistance is acquired at the sacrifice of Ms. In addition, Dy and Tb have low Clarke numbers, indicating that their resource amount is only a fraction of Nd, and are rarer than Nd. As a matter of course, the prices of Dy and Tb minerals are several times to ten times higher than that of Nd. The occurrence of these minerals is extremely biased to one country. From both the aspects of price and resource, the use of Dy and Tb becomes a neck for the Nd magnet manufacture from now on.
It is desirable to enhance the Hc of Nd magnets without substitution or addition of Dy and Tb, so that the Nd magnets may be used in a high-temperature environment above 100° C. The development effort capable of achieving this goal is important. Great investigations were made from both the composition and process sides, including substituting elements other than the above-listed Al, Cu and Ga, low-oxygen process, sintered structure grain refinement, and the like, and are now still continued. So far, the removal of Dy/Tb in magnet composition is not prospected, but a saving of Dy/Tb has been attempted by several proposals, some of which are approaching the practical level (WO 2006/64848).
With respect to the Dy/Tb saving, several different proposals are known, but they are common in that after preparation and machining of a sintered body, Dy/Tb is diffused and infiltrated into the body from the surface along grain boundaries. The resulting sintered magnet has the structure that Dy or Tb is localized only at and near major phase grain boundaries in a high concentration, and the concentration of Dy or Tb gradually decreases from the surface toward the magnet interior. Such a non-equilibrium structure is effective for Hc enhancement, because the coercivity mechanism of Nd magnet is of nucleation growth mode so that Hc is governed by the near grain boundary structure morphology and composition of the major phase (R is at least one rare earth element including essentially Nd, simply referred to as 2-14-1, hereinafter). Although any quantitative discussion on the nucleation growth mechanism is still impossible, it is true that Hc can be enhanced by magnetically strengthening only the near grain boundary structure with Dy or Tb. In addition, since these elements are localized only near grain boundaries, a decrease of saturation magnetization Ms is quite small as compared with the substitution of the overall alloy. The grain boundary localizing process reduces the amount of Dy or Tb used to acquire an identical Hc, to or below half of the amount of Dy or Tb used in the prior art for the substitution of the overall alloy during melting.
As described above, the Dy or Tb grain boundary localizing process is very advantageous from both the aspects of resource saving and magnetic enhancement. However, the process has some problems which are not critical, but too serious to be overlooked. One problem is that magnet machining must be followed by the extra step of diffusion or reprocessing for Hc enhancement. The increased number of steps, of course, increases the process expense. Since Dy/Tb diffuses from the magnet surface toward the interior along grain boundaries, a differential concentration of the element arises between the surface and the interior, resulting in a distribution of Hc within the magnet dependent on the distribution of the element. If the magnet thickness exceeds 10 millimeters (mm), for example, there is a possibility that the amount of Dy/Tb is zero at the center of the magnet. If the temperature and time of diffusion treatment are increased in order to flatten the concentration distribution between the surface and the interior, diffusion takes place deeper into the magnet interior, but the tendency of Dy/Tb diffusing from the grain boundary into the interior of major phase 2-14-1 grains becomes outstanding. This results in the same state as the addition of Dy/Tb during alloy preparation. For this reason, the thickness of a magnet to which the diffusion treatment is effectively applicable is at most several millimeters (mm). It is sometimes believed that in the case of motors and power generators, the enhancement of Hc only near the magnet surface where eddy current flows to generate an outstanding amount of heat is satisfactory. It depends on the use and quantity of Nd magnets from now on whether or not the distribution of He within the magnet becomes a rate determining factor in the magnet application.
Essentially desired is the removal of Dy/Tb. The Nd2Fe14B compound has an anisotropy field Ha (theoretical maximum coercivity) of about 6.4 MA/m (80 kOe). In contrast, sintered magnets of Dy/Tb-free Nd base composition have a He of about 0.8 MA/m at most. That is, only a He corresponding to about ⅛ of the theory is obtained. The qualitative description of He of Nd magnet is that the most disordered region (defect, transition, non-smooth surface, etc.) near boundaries of sintered major phase grains with a size of several microns (μm) to 10 μm becomes a bud of a reverse magnetic domain upon application of a reverse magnetic field, and magnetization inversion originates therefrom. It is true that the near grain boundary structure morphology of the major phase is related to Hc, but it is not evident what region or what component of the structure is a rate-determining factor of actual Hc, despite a vast amount of observations and investigations made thus far. Of course, at the present, investigations are concentrated on the control of grain boundaries and neighbors thereof, in order to clarify the He rate-determining factor. The difficulty of this measurement/analysis problem resides in the fact that a nanometer-order portion near the surface of grains with a micrometer-order size is a rate-determining factor to Hc, analysis must be made over the entire surface of a size of more than 1,000 times before the magnetically weakest portion becoming the He rate determining factor can be identified. There is available no method capable of analyzing on the nanometer order the entire three-dimensional surfaces of a sintered particle with a size of the micrometer order.
However, it is readily presumed from the results of the grain boundary localization method mentioned above that He is improved by tailoring the structure and composition near the surface of Nd magnet grains. For example, if a He of 1.6 MA/m which is approximately ¼ of the theoretical Ha is obtained, the majority of Nd magnet applications is covered. If a Hc of 2.1 MA/m which is ⅓ of the theoretical Ha is obtained, Dy/Tb addition is unnecessary except special applications. The current demand is exclusion of Dy/Tb rather than a saving of Dy/Tb.